This application claims the priority of German Patent Application Ser. No. 198 42 042.0, filed Sep. 14, 1998, the subject matter of which is incorporated herein by reference.
The present invention relates to a screening device for screening fiber suspensions, and more particularly, to a screening device which can be applied in the paper industry, for example with pressurized sorters.
Screening devices typically include a substantially cylindrical screening element with a screening surface in the form of a slotted hole screen mat or a screen plate provided with drilled or milled/sawn screening slots. Such screening device has to withstand high static and dynamic loads, particularly in sorter applications. Hence, a spacer element is typically associated with the cylindrical screening element. The cylindrical screening element with the spacer element is supported on a reinforcing element with a radial clearance. The reinforcing element can have the form of, for example, a reinforcing jacket. Terminal elements, for example, in the form of end rings, can be provided on the axial ends of the screening device to aid in positioning of the screening device. In typical arrangements, the respective terminal element is welded to the reinforcing element and the screening element, whereby the screening element is assembled first and the reinforcing element is subsequently positioned from the outside over the screening element and thereafter secured. Finally, the ring-shaped terminal elements are connected with the cylindrical screening element along the circumference of the reinforcing element. The welding seams of the weld connection are accessible only from one side. The high static and dynamic loads can introduce notch stress, so that the durability of such screening device depends strongly on the integrity of the weld connection.
In typical applications, the screening element in particular is subjected to severe wear, with the reinforcing element having a much longer service life. Because the manufacture of the screening device requires a weld connection between the screening element with the spacer element and the reinforcing element, the entire screening device has to be exchanged when the screening element wears out, which is expensive and complex.
European Pat. No. EP-0 471 195 A1 as well as U.S. Pat. No. 5,200,072 discloses screen plates, screen cylinders and methods for manufacturing the same. The cylindrical screening device is designed so that the screening element and the screening units forming the screening element are releaseably and interchangeably connected to a support cylinder that forms the reinforcing element. The manufacturing cost and the manufacturing time is reduced in that only a worn-out screening element has to be replaced, whereas the reinforcing element, in particular the support cylinder, can be reused. The releaseable connection between the screening element and the jacket-shaped reinforcing element is formed by making the outside diameter of the screening element slightly greater than the inside diameter of the support cylinder. The screening element and the reinforcing element are non-positively secured to one another by a temperature-dependent shrink-on process. This connection can optionally be secured by an additional step, for example, by welding, riveting, with screws, adhesives, solder and the like.
U.S. Pat. Nos. 4,954,249 and 5,023,986 disclose a modular screen basket with several small ring-shaped cylindrical screening elements. The individual cylindrical screening modules are interchangeable and are positively connected with reinforcing rings via notches. The axial end rings are tensioned with respect to one another by a plurality of tension rods. The axial tension rods are simultaneously guided through bores provided in the reinforcing rings.
European Pat. No. EP-A-0 724 037 discloses a screen basket wherein the profile rods are positively connected with reinforcing rings.
German Pat. No. DE 39 42 484 A1 describes a sorter with a screen basket which is supported on the sorter housing by support rings. The design of the support structure is quite complex and the axially spaced apart support locations can absorb only radial forces, but no torsion forces. Since the sorter housing provides the main support function, it requires modifications to accommodate different types of screen baskets or screening elements, since there are no additional reinforcing elements aside from the sorter housing that can be adapted to support the screen basket.
J and L Fiber Services, Inc., Waukesma, USA, supplies screening cylinders, wherein a plurality of short rod-shaped elements is positively located in grooves of reinforcing rings and fixed in position by tension rods, which is time-consuming and expensive. The entire screening cylinder has to be disassembled to exchange any damaged and worn elements.
It would therefore be desirable to provide a screening device of simple design which can be applied, in particular, with fiber suspensions in the paper industry, and which permits an economical and cost-effective reuse of those elements of the screening device that are not severely worn, such as the reinforcing element. In addition, the screening element and the reinforcing element should be reliably connected to one another so as to support both dynamic and static loads.
According to one aspect of the invention, a screening device, in particular for fiber suspensions in the paper industry, is provided which has a substantially cylindrical screening element forming a screening surface, with the screening element being radially supported on a reinforcing element by spacers with clearance therebetween. The screening element can be interchanged as one unit and is non-positively secured to the reinforcing element by a frictionally engaging clamping connection.
In this way, the screening element forming the screening surface can be released from the reinforcing element when the screening element is consumed. The reinforcing element, on the other hand, can be reused with the screening device after insertion of a new screening element. The design of the screening device according to the invention is based on the observation that the actual screening and/or sorting function is mainly performed by the screening element with the screening surface, possibly in cooperation with the spacer elements, whereas the reinforcing element provides the actual support for the screening element and facilitates insertion of the screening device in the respective machine. The clamping connection between the screening element and the spacer elements, on one hand, and the reinforcing element, on the other hand, distributes the clamping force uniformly over the entire circumference of the screening device and the reinforcing device. The clamping forces of the frictionally engaging clamping connection alone can reliably secure the screening element against rotation with respect to the reinforcing element. Since this connection is releaseable, the screening device can be removed from the reinforcing device without damaging the reinforcing device. Accordingly, the invention permits the reinforcing element to be reused repeatedly, while the screening element itself, which is subject to wear even if the screening device is used as intended, can be exchanged as an entire unit.
Advantageously, the entire screening device can be pre-fabricated and unit exchanged as a single unit. In particular, the reinforcing element, which can be fabricated separately and independently of the screening device, provides the actual support function and ensures the proper fit of the screening device to the machine, such as a sorter. By separating the screening and support functions and/or the fit, the respective components of the screening device, i.e., the screening element and/or the reinforcing element, can be designed and constructed to take into account the desired properties, without requiring tradeoffs for the purpose of a matching these elements.
The screening device of the invention therefore obviates the rigid connection between materials, such as a weld connection between the reinforcing element and the screening element. By making those screening elements that tend to wear out sooner interchangeable and by reusing the reinforcing element, the screening device of the invention uses less material. Moreover, the screening device can be procured cost-effectively and the components that are not consumed, can be reused. The screening devices according to the invention can also be reconditioned and returned to service much more quickly, because not all the fabrication steps that are required for the initial production of the screening device have to be executed, so that some of these steps can be omitted. The screening device of the invention can be modular, facilitating its fabrication and stocking. For example, screening elements as well as associated reinforcing elements can be pre-fabricated in standard sizes and assembled application-specific to a screening device. The screening device of the invention also allows cost-effective retooling, if a screening device of a different design should be used, for example, to improve efficiency, even if the screening device itself is not yet worn out and is still be usable.
Another aspect of the invention is the particular design of the releaseable connection between the actual screening element and the reinforcing element. The frictionally engaging clamping connection provided by the invention secures the screening element with the substantially cylindrical screening surface reliably and firmly along the entire circumference of the reinforcing device.
According to another feature of the present invention, the clamping connection secures the screening element through frictional engagement, preferably through a wedge action on the reinforcing element. For this purpose, non-positive clamping devices and/or clamping elements are employed, preferably with a wedge action for providing the non-positive connection.
Alternatively, the clamping connection can be implemented so that the screening element is secured on the reinforcing element through a conical seating action in the axial direction. With this arrangement, the outside surfaces of the spacer elements of the screening elements with the screening surface, which have a conical shape in the axial direction, cooperate with a corresponding conically shaped reinforcing element to provide a non-positive connection. In this arrangement, the clamping connection between the screening elements with spacer elements and the reinforcing element is implemented as an axial conical press fit. In other words, this screening element forms a screening insert with a substantially cylindrical inside surface as the screening surface and a conically shaped axial outside surface formed by spacer elements. The screening device is formed by inserting the screening insert in a corresponding conical reinforcing element, for example a reinforcing jacket, extending in the axial direction. The conically engaging seating surfaces provide the clamping connection between the screening device and screening insert, respectively, and the reinforcing device.
Advantageously, the clamping connection can include clamping elements which are radially displaceable towards the reinforcing element for forming the clamping connection. In this way, the type, number and arrangement of the clamping element that provide the clamping connection can be selected to conform with the specific requirements, such as the size and application and the like.
According to another feature of the present invention, the clamping connection may include clamping elements with an I-shaped cross-section. One of the transverse sides contacts the screening device, with the other transverse side contacting to the spacer elements. Preferably, such clamping element can be in the form of a strip extending in the axial direction of the screening device. The screening element with the screening surface and the spacer elements are here tensioned with respect to the reinforcing element such that the screening sections of the screening surface with the associated spacer elements are spread apart in the circumferential direction by the clamping element and tensioned against the reinforcing element. By forming the clamping element as a strip extending in the axial direction, the clamping strip can be driven in the axial direction from one end face to the other end face of the screening element between the screening sections of the screening element, thereby spreading and tensioning the screening element against the reinforcing element.
According to another feature of the present invention, the clamping elements may be configured in the form of wedge elements with a radial tensioning direction to establish the clamping connection. Such wedge-operated tensioning connection can be formed through respective cooperating wedge and spacer elements by forming mutually cooperating inclined wedge tensioning surfaces. The respective wedge elements can be implemented in many different ways.
According to another feature of the present invention, the clamping element may have be T-shaped configuration, with the transverse side of the xe2x80x9cTxe2x80x9d contacting the screening device. Advantageously, the T-shaped clamping element of this embodiment can be in the form of a rail or strip extending in the axial direction of the screening device or inclined thereto or can have a helical shape, thereby producing a predetermined two-dimensional bias force for the desired clamping connection of the screening device.
The T-shaped clamping element can be implemented as two separate parts, with the transverse side section contacting the screening surface of the screening element and a longitudinal side portion penetrating the spacer element. For example, a screw connection can be employed for non-positively securing and tensioning the reinforcing element. In this way, the non-positive connection can be produced by using a variety of prefabricated components and even standard parts, thereby reducing manufacturing costs.
The clamping elements of the clamping connection can be displaced in the radial direction using at least one tensioning element. The tensioning element can cooperate with the clamping element and be in the form of a screw, rivet or bolt connection adapted to radially move the clamping element.
Advantageously, the respective tensioning element can be secured to the reinforcing element after the clamping connection has been established. Different attachment methods, such as welding and the like, can be used. Optionally, a post treatment can be employed wherein, for example, the tensioning elements attached to the reinforcing device can be ground in the transition regions or finished in other ways. If necessary, partition lines produced during manufacture of the clamping connection can be smoothly fused together.
The tensioning element of the screening device extends at least partially, preferably completely, through the reinforcing element in the radial direction. A force applied to the outside of the reinforcing element establishes the clamping connection.
According to another feature of the present invention, a terminal element, for example in the shape of a ring, such as an end ring, can be releaseably connected with the reinforcing element for axially affixing the screening element on at least one axial end face of the screening element to allow positioning of the screening element in the axial direction. The terminal element can be designed as a type of adapter unit to match the characteristic of the machine. With this three-part design consisting of a screening device, a reinforcing device and terminal elements on the end face, the reinforcing device can be constructed, for example, in the form of a reinforcing jacket, regardless of how the screening device cooperates with the machine, thereby simplifying its manufacture. The terminal elements or rings perform the function of adapters and are fitted to the specific use of the machine, such as a sorter.
The releaseable connection between the terminal element(s) and reinforcing element(s) can also include pins, screws and other types of fasteners.
According to another feature of the present invention, the substantially cylindrical screening surface of the screening element can be formed of a slotted screening mat and/or a screen plate and/or sections thereof. The screening device can use conventional screening elements and components, such as slotted screening mats or curved screening plates, into which corresponding screen openings can be cut. The screening elements can also have helical screening slits or slots disposed on a screening surface formed by the screening element.
The reinforcing element can be formed as a reinforcing jacket provided with openings, also called a backup jacket. To strengthen the screening element against radial pressure loads, the reinforcing element can include additional reinforcing rings disposed on its outside surface.
According to another feature of the present invention, the terminal element can be welded to the reinforcing element while the reinforcing element is accessible from both sides and before the screening device with the spacer elements is attached, if the screening device includes at least one terminal element, for example in the form of an end ring, arranged on the axial end face(s). The screening device can then be assembled from the individual components from the inside outwardly, while the terminal element and the reinforcing element are accessible from both sides when the weld seam is formed, thereby ensuring a high-quality weld seam that can withstand high stress. In addition, the two-sided access reduces notch effects on the weld seam. The screening device with the spacer elements is installed after the connection between the terminal elements and reinforcing elements is formed. Subsequently, the screening device with the spacer elements and the reinforcing element are secured to one another by a non-positive connection. With this arrangement, the screening device can be installed and removed of as a single unit.
According to another feature of the present invention, the screening element with the spacer elements can be located relative to the terminal element in the axial direction by interposing an insertion strip, which obviates the need for an additional weld connection to affix and axially position the screening device and the spacer elements. This further simplifies the installation and removal, with the insertion strip effectively compensating tolerances in the axial direction of the screening device.
According to another feature of the present invention, a pin arrangement is employed to effectively transmit shear forces directly to the reinforcing device, in particular during operation, and prevent the screening element with the spacer elements from being displaced relative to the reinforcing element in the circumferential direction even under a severe tangential load. This arrangement reduces the stress on connections that are susceptive to shear forces, such as screw connections and weld connections. In particular, the screening element is thereby prevented from rotating relative to the reinforcing element, providing a permanent non-positive connection for the screening device while at the same time eliminating shear forces that can adversely affect the non-positive connection.
The present invention simplifies the design of the screening device by allowing those components of the screening device that are not subject to severe wear to be easily exchanged and reused cost-effectively. The screening element together with the spacer elements is secured to the reinforcing device by way of a frictionally engaging clamping connection, without permanently joining the materials, so that the screening element can be exchanged without destroying or disassembling the entire screening device. Any optional terminal elements disposed on the axial ends of the screening device are permanently connected only with the reinforcing device, and not with the screening device and the spacer elements. Hence, the screening to elements can be replaced, for example, by partially disassembling the screening device, while leaving the original connection between terminal elements and reinforcing element in place.